1. Field of the Invention
The present invention relates to methods for forming solid and semi-solid clathrate hydrate structures useful for removing salt and other soluble materials from water. In particular, a series of completion steps involving pressurization and clathrate formation, depressurization, ice formation and desalting is used to recover the desalted water from the clathrate hydrate structures.
2. Description of Prior Art
Produced water is the raw water that is a byproduct of natural gas and oil production. Produced water contains organic matter, suspended silts and clays, and formation brine. It is commonly re-injected below ground due to the high cost of purification for above ground disposal. In particular, the cost of separating the dissolved formation salts from the water is expensive. An economical method to purify produced water would be of great benefit to oil and natural gas producers, particularly in regions where oil and gas are produced and surface irrigation or stream water is in demand.
Common techniques for the desalination of water include distillation, reverse osmosis and freezing. Distillation involves the evaporation of saline water and collection of the freshwater condensate. Reverse osmosis requires high-pressure pumps to force saltwater through a semi-permeable membrane to produce freshwater. This technique is more expensive due to the costs of membranes and pumps.
Various techniques are used to produce freshwater using a freezing process. Fundamentally, the freezing process relies on the fact that salt is rejected from the ice. In typical freezing processes, cooling elements are located in saltwater. When ice forms on the cooling elements, they are separated from the saltwater, melted, and freshwater is produced.
Hydrate formation is being developed as a desalination technique for use in oceans (McCormack, Ref. 2), (Max, Refs. 3, 5). Such techniques involve the use of a long tube (˜100 meters), or a desalination fraction column that is situated to transect the hydrate stability zone, thereby allowing hydrates to form and dissociate in the column. The hydrate forms at the bottom of the column and rises through the column due to its buoyant properties. As the hydrate rises, it crosses into its zone of instability and dissociates to produce freshwater. The freshwater collects at the top portion of the column due to density differences between freshwater and saltwater.
It is also known that solid or semi-solid clathrate hydrate structures can be formed by mixing water and certain gases (gas hydrates) such as carbon dioxide and methane. As the clathrate hydrate structures form, they tend to exclude salt or any dissolved species from the clathrate structure (Donath, Ref. 1), (Knox, Ref. 6). The present invention takes advantage of this feature in a series of completion steps that recover the desalted water.
A technology called coflow injection has been developed at the Oak Ridge National Laboratory for continuous hydrate production. It involves high energy mixing of a hydrate forming fluid and water in a coflow injector, and then ejecting the mixture into a pressurized vessel (West, Refs. 4, 7), (Lee, Ref. 8). It has been observed that when the injected water has solid particles suspended or dissolved in it, the hydrate formation process appears to exclude the solids from the solid hydrate composite.
An advantage of the coflow injection method over conventional batch reactor type hydrate producers is that the injector may be used for continuous hydrate production, and the shape of the produced solid material allows for easier transfer of excluded solids and dissolved species. A further advantage of the coflow injector is the demonstrated ability to precisely control the density.